1. Field of the Invention
The present invention relates to a liquid crystal display device, and more specifically, to a liquid crystal display device capable of producing an image with uniform brightness at various viewing angles.
2. Description of the Prior Art
Since a liquid crystal display device (LCD) has the advantages of better portability, lower power consumption, and lower radiation, the LCD is widely used in various portable products, such as notebooks, personal data assistants (PDA), and video cameras, and the LCD replaces a conventional CRT monitor gradually.
It is well known that incident light will produce different polarization or refraction when the alignments of liquid crystal molecules are different. Accordingly, the LCD utilizes the liquid crystal molecules with different alignments to control the light transmittance of each pixel unit so as to generate light beams with different intensities of gray level or to generate red, blue, and green lights with different brightness, so that the LCD is enabled to produce gorgeous images. Please refer to FIG. 1. FIG. 1 is a sectional view of a prior art liquid crystal display device. As shown in FIG. 1, a liquid crystal display device (LCD) 10 comprises a liquid crystal display panel 12 and a backlight module 20 positioned below the liquid crystal display panel 12. Generally, the LCD 10 further includes an upper polarizer (not shown) positioned on an upper surface of the liquid crystal display panel 12, and a lower polarizer (not shown) located between the liquid crystal display panel 12 and the backlight module 20.
Additionally, the liquid crystal display panel 12 has an upper substrate 14a, a lower substrate 14b, and a liquid crystal layer 18 sealed between the upper substrate 14a and the lower substrate 14b. The liquid crystal display panel 12 further includes transparent conductive layers 16a and 16b respectively formed on a lower surface of the upper substrate 14a and an upper surface of the lower substrate 14b. When the conductive layers 16a and 16b are applied with a voltage, the alignments of the liquid crystal molecules can be adjusted to control the light transmittance of each pixel unit. In addition, the backlight module 20 at least comprises a light source 20a for generating light beams, a light-guiding plate 20b for guiding the light beams, a diffuser 22 for equally distributing the light beams, and a reflective plate 24 for reflecting light beams into the liquid crystal display panel 12. Usually, the transparent conductive layers 16a and 16b both can be composed of indium tin oxide (ITO). The light source 20a can be a light emitting diode (LED) or a cold cathode fluorescent lamp (CCFL), and the light-guiding plate 20b is usually made of acrylic.
As shown in FIG. 1, the light beams generated from the light source 20a are firstly reflected by spots on a bottom of the light-guiding plate 20b, and then, the reflected light beams are ejected from the light-guiding plate 20b and enter the liquid crystal display panel 12. Therefore, the LCD 10 can display various images with the aids of the reflected light beams and other optical units, such as polarizers or color filters. Additionally, when a user watches the images displayed by the LCD 10, an intersecting angle between eyes of the user and the axis 26 that is perpendicular to a surface of the LCD 10 is defined as a viewing angle θ.
Referring to FIG. 2, FIG. 2 is a simplified schematic diagram illustrating the relationship between the luminance provided by a backlight module and a viewing angle (θ). As shown in FIG. 2, the backlight module 20 generates light beams with uniform brightness at each viewing angle. Accordingly, the backlight module 20 can function to supply light beams with sufficient brightness and uniform distribution so that the LCD 10 can display high quality images.
Referring to FIG. 3 and FIG. 4, FIG. 3 is a simplified schematic diagram illustrating the relationship between the light transmittance of the LCD 10 and a viewing angle (θ). FIG. 4 is a simplified schematic diagram illustrating the relationship between the luminance displayed by the LCD 10 and a viewing angle (θ). As shown in FIG. 3, the light transmittance of the liquid crystal display panel 12 gets smaller as the viewing angle becomes larger. Accordingly, even though the backlight module 20 can provide light beams with uniform brightness for the liquid crystal display panel 12, the brightness displayed by the LCD 10 still decreases as the viewing angle increases, as shown in FIG. 4. That is to say, as the user watches the images displayed by the LCD 10, the brightness observed by the user varies as the user changes his/her position, which prevents the user from enjoying the images displayed by the LCD 10 comfortably and easily.